Continuous fluid treatment apparatus include tube reactors in which wet oxidation or other chemical reactions occur in a fluid waste stream. For example, in U.S. Pat. No. 4,272,383 to Dr. Jay L. McGrew, entitled "Method and Apparatus for Effecting Subsurface, Controlled, Accelerated Chemical Reactions" (assigned to the assignee of the present invention), a vertical-tube reaction apparatus is disclosed that is the basis for the first successful verticaltube, subterranean, wet oxidation reaction system for municipal waste sludge, operated at Longmont, Colo. The aformentioned McGrew patent, which is expressly incorporated herein by reference, discloses an assembly of concentric pipes or tubes which extend into the earth to a depth of about 5,000 feet. These generally concentric tubes form separate vertical passages for an influent stream and an effluent stream in annuluses defined by the tube walls. Municipal waste sludge, or other fluid waste, is circulated through the apparatus by injection into the influent stream passage through which the waste flows downwardly in plug flow to a reaction zone at a depth of preferably between 3,000 to 6,000 feet, forming a hydrostatic column of considerable fluid pressure. In the reaction zone, a controlled, exothennic chemical reaction occurs which generates substantial heat. Reactor heat can be controlled with a heat-transfer jacket adjacent the reaction zone. The reaction products and any unreacted materials are then caused to flow up through the effluent stream passage and out of the reactor. The assignee of the instant application is also experimenting with above-ground plug flow tube reactors wherein the fluid waste is circulated in a generally circular path under pressure as disclosed in a co-pending application, wherein scaling problems are anticipated. One of the primary chemical reactions in processing municipal waste sludge for example is the wet oxidation of organic materials to produce simpler compounds that are more environmentally desirable. Municipal waste sludge commonly includes sulfur-containing organic compounds which are oxidized in a wet oxidation reaction to form sulfate ions. However, these sulfate ions tend to combine with ions of calcium, magnesium and aluminum, present as salts in the sludge, to form a scale which builds up on the wall surfaces of the reactor tubes.
In particular, anhydrous calcium sulfate, also referred to herein as anhydrite, forms when calcium and sulfate ions combine in the waste reactor as waste sludge moves through the reaction zone. It is known that the solubility of anhydrite decreases as temperature increases. Temperatures in the reactor generally exceed 300.degree. F. and preferably are in excess of 500.degree. F. during operation. Hence, anhydrite scale rapidly forms on the hot metal reactor surfaces. In addition to anhydrite scale, other types of scale form on the reactor tube walls as a result of other ions such as magnesium and aluminum which may also be present in municipal waste sludge. In the unique environment of the tube reactor, the accumulation of anhydrite and other scales on the walls of the reactor is a serious problem which reduces the efficiency of heat transfer and inhibits the flow of waste materials through the reactor tubes. In order to remove this unwanted build-up of scale, the reactor must be shut down for as long as twenty-four hours to permit the laborious removal of scale by nitric acid washes and the like. It would therefore be desirable to prevent to the extent possible the build-up of scale on the walls of the reactor.
Known scale inhibitors are not effective for use in inhibiting scale formation in high temperature environments such as a vertical tube waste reactor. These conventional compositions are generally organic compounds which for the most part are ineffective at temperatures above 300.degree. F. As is known, organic scale inhibitors undergo thermal decomposition at high temperatures. Therefore, conventional organic scale inhibitors are unsuitable for use in preventing build-up of unwanted scale on the walls of tube reactors.
Thus, there is a need for a simple and effective process for inhibiting or mitigating scale formation on the walls of tube reactors so that the waste reaction process can continue uninterrupted by frequent de-scaling operations.